Work in this laboratory had revealed information about promoter, enhancer and repressor elements that regulate human cytomegalovirus (HCMV) genes and the relationship between gene expression and binding of transcription factors to cisacting elements. In extension of this work, we have examined transcriptional regulation from the HIV-1 long terminal repeat (LTR) which contains control elements required for viral activation. In vitro transcription systems were established from lymphoid and nonlymphoid cell lines and accurate initiation of transcription from the HIV LTR was observed. New findings show that the in vitro systems can direct enhancer-dependent transcription from the HIV-I LTR in several cell types, including T-cells, B-cells and epithelial cells (1). Although regulatory elements within the enhancer core sequences are preferentially active in lymphoid cells, only a limited host cell restriction of HIV transcription initiation in vitro was observed (1). The possibility of transcription factors binding to the enhancer sequences and mediating transcriptional stimulation was investigated. Using sensitive binding and competition assays it was shown that proteins bind to the HIV-1 enhancer core sequences (1) and that protein-DNA irteraction is necessary for enhancer dependent transcriptional stimulation (2).